1. Field of the Invention
The present invention relates to a lead-free low softening point glass in the system SnO—P2O5.
2. Description of the Background Art
Heretofore, a low softening point glass in the system PbO—B2O3—SiO2 has been widely used as a primary composition of adhesives for glass, ceramic or metal materials; sealing or coating glasses for electronic components; or conductive or resistive pastes. In late years, it has also been used as a material of the glass substance for a plasma display panel (PDP) or a vacuum fluorescent display (VFD).
In this context, the recent concern over the risk of lead toxicity to the human body develops a tendency to restrict the use of PbO contained in the low softening point glass.
As lead-free low softening point glasses substituting for the PbO-containing glass, there have been known BaO—B2O3—ZnO glass system as proposed in the Journal of the Japan Institute of Metals, Vol. 63 (1999), pp 284, and Bi2O3—B2O3—SiO2 glass system. U.S. Pat. Nos. 5,246,890 and 5,281,560 also disclose ZnO—SnO—P2O5 glass system.
Among these lead-free low softening point glasses, the BaO—B2O3—ZnO glass system and Bi2O3—B2O3—SiO2 glass system have a problem on a higher softening point than that of the conventional lead-containing glass. Thus, these glasses have to rely on the addition of an alkali metal oxide (R2O) to obtain the same level of softening point as that of the conventional lead-containing glass, and the addition of R2O is likely to cause side issues in terms of water resistance, thermal expansion properties and/or electric properties. In addition, some of the BaO or Bi2O3-containing glasses are potentially harmful to the human body.
The ZnO—SnO—P2O5 glass system with a softening point equal to or lower than that of conventional lead-containing glass have been regarded as a dominant material among the lead-free low softening point glasses.
On the other hand, the ZnO—SnO—P2O5 system glasses involve a problem peculiar to a phosphate-based glass, such as poor water resisting property or large coefficient of thermal expansion. Furthermore, SnO is easily oxidized to form SnO2, and thereby unmeltable tin-phosphate compound (SnP2O7) are apt to be undesirably formed during the glass melting process under an ambient atmosphere.
While a trivalent cation oxide, such as aluminum oxide (Al2O3), iron oxide (Fe2O3), gallium oxide (Ga2O3) or antimony oxide (Sb2O3), may be added thereto to improve the water resisting property, the addition of such an oxide will cause a new problem of the deterioration in low softening point as the prime advantage of this system. As a measure for reducing the coefficient of thermal expansion, a filler with a coefficient of thermal expansion lower than that of the glass may be added to the glass at an amount proportional to the level of the coefficient of thermal expansion thereof. However, if the glass has a fairly high coefficient of thermal expansion, the filler added at a larger amount will cause the need for increasing the temperature of a heat treatment, and consequently spoil the advantage of the low softening point. Further, in order to suppress the formation of tin-phosphate compound, the melting process has to be performed under the condition of an inert atmosphere, a two-stage temperature control and/or the addition of a reducing agent. The resulting complicated melting process will lead to instability in the production process.
Japanese Patent Laid-Open publication No. 09-235136 also discloses a lead-free low-softening-point glass composition for use in sealing PDP or VFD, comprising 0 to 5 mol % of MnO contained in phosphorous and tin oxides. In this publication, the MnO is added together with CuO, NiO, CoO, Fe2O3, Bi2O3 and others to provide enhanced bonding strength. That is, it is not intended to add the MnO at a sufficient amount (mol %) as an essential component for directly contributing to improvement in water resistance and coefficient of thermal expansion.